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Technical Manual
137
11.25 Heater Circuit
Please refer to diagram on page 138.
1.
The water temperature in the heater tank and pressure boiler(B2C
machines) is controlled by the thermistor probe. The thermistor has a
variable resistance - when cold it has a high resistance and when hot a low
resistance.
2.
The thermistor probe sits directly in the water and continuously senses
the water temperature. The resistance of the thermistor is interpreted by
the controller as a temperature reading.
N.B. The resistance of the thermistor when at ambient (room)
temperature should read approximately 3000 ohms. When hot (96°C) it
should read approximately 220 ohms.
3.
If the water needs to be heated, a signal from the controller is sent down
the Comms. link to the I/O board (heater on signal). The I/O board then
switches a 24 volt negative output to the solid state relay. This in turn
switches a 240 volt supply to the heater element which heats the water.
N.B. The heater element fitted to the heater tank is rated at 2.4kW. The
element fitted in the pressure boiler (B2C machines only) is rated at 2kW.
4.
This process continues until the water has reached the temperature set in
the temperature configuration program.
5.
If the water in the heater tank should overheat and boil over, a high
temperature cut out, positioned in the overflow pipe, will cut off the mains
supply to the heater at approximately 90°C within 60 seconds.
6.
When the maximum temperature (set in the temperature configuration
program) has been reached the ‘heater on’ signal is removed from the
Comms. link switching off the 24 volt negative output from the I/O board,
the solid state relay and the heater element.
7.
B2C Machines: Whenever an espresso based drink is selected, the water
is heated in the pressure boiler throughout the vend.